The present invention relates to nonlinear optical materials applicable in various optical wavelength converters, a method of manufacturing the same, and wavelength converters using the same.
As photonics materials useful in future optical devices, materials showing great nonlinear optical effects and responding at high speed are in demand, and widely searched for.
As such materials, as compared with inorganic compound crystals to which the vibration of a crystal lattice is related, organic compounds possessing a .pi.-electron conjugated system are said to be superior in response and magnitude of threshold of optical damage from intense light such as laser light. As the design principle, it was hitherto effective and routine to introduce a strong electron attractive group and a strong electron donative group in the molecule possessing a .pi.-electron conjugated system. As organic materials possessing great nonlinear optical effects, nitroaniline derivatives possessing a nitro group and an amine group as substituent groups are generally known. Moreover, nonlinear optical materials possessing an intra-molecular charge transfer structure without an inversion center, while having a large hyperpolarizability .beta., have been reported (Japanese Patent Application Laid-open No. Hei. 3-95147). Using these single crystals, it has also been attempted to oscillate a solid laser by semiconductor laser pumping, and to fabricate a wavelength converter for converting the wavelength of the light, for example, as disclosed by Kitaoka, et al., in Extended abstracts 30-Q-13 (the 37th spring meeting, 1990), the Japan Society of Applied Physics and Related Societies, and Y. Goto, et al., Proceeding of SPIE, 1337, 297 (1990).
For excellent nonlinear optical materials at the molecular level, compounds are desired to have a large second order hyperpolarizability .beta. those effects appear in a strong electric field of light; and in an assembled form of crystal, it is desired not to possess the inversion center.
However, in order to demonstrate the nonlinear optical characteristics sufficiently, if an attempt is made to enhance the second order hyperpolarizability by introducing a strong electron attractive group and an electron donative group the organic compound possessing a .pi.-electron conjugated system, the crystallization is difficult. Even if crystals are formed, the crystallization occurs in such a manner as to mutually cancel the intrinsic large dipole moments, and nonlinear optical characteristics are not demonstrated at the crystal level. Moreover, to generate the second order nonlinear optical effect, when a bulky substituent is introduced so that the crystal may not possess a center of symmetry, although large nonlinear optical characteristics are exhibited in the powdered micro crystal state, it could not be used as wavelength converter because large crystals were not obtained.
As an other problem of such compounds, they are unstable thermally because the melting point is low, or vapor pressure is high and the sublimation property is high. Yet, by introducing a strong electron attractive group and an electron donative group in order to obtain large nonlinear optical characteristics, the absorption spectrum of the compound is extended to the longer wavelength side, and hence in the wavelength conversion of semiconductor laser light in the near infrared wavelength region, the fundamental wave and generated second order harmonics are absorbed; thus the desired second order harmonics cannot be taken out efficiently.